FIG. 8 is a block diagram showing a principle of a discharge machining apparatus based on the conventional technology. In FIG. 8, designated at the reference numeral 21 a pipe-shaped electrode, at 22 a workpiece to be machined by means of discharge machining, at 23 a machining power supply unit for supplying a power, at 24 a rotating device for rotating the electrode 21, at 27 a short-circuit detecting circuit for detecting generation of a short-circuit between the electrodes, and at 28 a short-circuit back control circuit for executing a short-circuit evading operation for the electrode 21 with the short-circuit detecting circuit 27. Also, H indicates a machining feed of the electrode 21 in the X- and Y-axial directions, and V indicates a feed in the Z-axial direction for compensating depletion of the electrode 21.
Next description is made for operations. In the discharge machining operation shown in FIG. 8, machining is executed in a state where a shape of the electrode is kept under stable conditions by applying a voltage to a space between the electrodes 21 mutually opposing to each other and the workpiece 22, and synthesizing a Z-axial direction component feed V for compensating a depletion rate of the electrode in the longitudinal direction thereof and X- or Y-axial direction component feed H. As a result, a constant shape can be obtained without compensating depletion of the electrode in the sideward direction.
In the discharge machining apparatus described above, when machining is started, the electrode 21 is rotated by the rotating device 24 to execute machining, but in a case where of the electrode 21 having an especially small diameter is maintained by such a device as a collet chuck, axial deflection of the electrode 21 easily occurs in the horizontal direction as shown in FIG. 9A and FIG. 9B. Namely, assuming that a periphery of the electrode 21 rotating in a state where axial deflection has not occurred, the peripheral is widened up to W2 because it displaces to the outer side because of axial deflection by e as shown in the figure. Actually, as shown in FIG. 10, axial deflection occurs in the directions of points A, B, C, and D. Because of this axial deflection, a short-circuit is generated by this axial deflection between the electrode 21 and the work 22.
FIG. 11 is a view plotted with generation of short-circuit due to axial deflection on the time axis. Generation of each short-circuit is detected by the short-circuit detecting circuit 27. When Ts (t) becomes longer than a prespecified period of time in the state where a short-circuit is generated as shown at point B in FIG. 10, the short-circuit back control circuit 28 executes a short-circuit back operation for evading a short-circuit.
Generally it is difficult to suppress axial deflection to zero during machining with a rotating electrode, and a time zone Ts, in which a short-circuit is continued during one rotation, is generated. When a speed of electrode rotation is filly high (several thousands to several tens of thousands rotations/min or more), the short-circuit duration time Ts becomes shorter than the short-circuit determination reference time, and a short-circuit evading operation is not executed.
Namely feeds V, H of the electrode 21 are controlled by an ordinary inter-polar servo. On the other hand, when the rotational speed is low (several tens to several hundreds rotations/min), the short-circuit duration time Ts becomes longer than the short-circuit determination reference time, so that a short-circuit evading operation or a short-circuit back operation is executed in each rotation. In the state as described above, machining enters an extremely unstable hunting state with the machining speed remarkably lowered.
The discharge machining apparatus based on the conventional technology has the configuration as described above, and when the rotational speed is low, a short-circuit evading operation, namely a short-circuit back operation is executed in each rotation of the electrode, and machining enters into a hunting state, and the machining speed is disadvantageously lowered. To overcome this problem, it is necessary to increase the rotational speed, but in this case cost of the rotating unit becomes expensive, and the machine may deform due to heat emission when rotated at a high speed, which in turn, for instance, remarkably lowers the machining precision.